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Number 561, October 15, 2001 by Phil Schewe, James Riordon, and Ben Stein

CO₂ Production at the Single-Molecule Level

CO₂ production at the single-molecule level has been observed for a specific chemical reaction. Rather than micromanage (or nanomanage) compliance of the Kyoto Treaty, this new accomplishment instead offers fundamental chemical details of CO₂ formation which may bring improvements to automobile emission control, air purification, and chemical sensing.

Using a scanning tunneling microscope (STM) as a "nanoreactor," researchers (Wilson Ho, UC-Irvine, 949-824-5234, wilsonho@uci.edu) studied the oxidation of a single carbon monoxide (CO) molecule on a metal surface. In this "catalytic oxidation reaction," CO combines with oxygen (O) on the surface to form CO₂.

Putting CO next to two oxygen atoms on a silver surface, the researchers found that the CO and O species must be very close to each other to react. When the CO is at the closest possible distance to an oxygen atom on the surface, at just 1.78 angstroms away, the researchers saw that the reactants form an intermediate O-CO-O complex, which has not been observed to date (see images). Then, tunneling electrons from the STM tip flow through the CO and O species to bring the O-CO-O complex to a higher energy state and enable it to transform into CO₂ and a lone oxygen atom. (In a real-world situation, heat rather than tunneling electrons would spur this transformation.)

In separate experiments investigating another pathway for the same chemical reaction, the researchers put a CO molecule on the STM tip, and then positioned it close to an O atom on a silver surface. In this case, the CO and O loosen their respective bonds to the STM tip and the silver surface. Eventually, both species are on the surface and they interact to form CO₂. In demonstrating this, they showed that CO and O don't initially have to be on the surface to form CO₂; the CO can come from above. The researchers also ruled out the idea that CO reacts with molecular oxygen (O₂) on the surface to form CO₂; instead it only reacts with atomic oxygen. (Hahn and Ho, Physical Review Letters, 15 October 2001.)

Sticky and Slippery Nanobubbles

The presence of bubbles only twenty to thirty nanometers tall appears to explain why some surfaces can be surprisingly slick or unusually sticky. The bubbles form on the surface of hydrophobic (water repellent) materials immersed in water. The stickiness arises as two hydrophobic surfaces approach one another and the bubbles link them together, leading to an attractive force with a range related to nanobubble height. Alternatively, the bubbles can serve as a kind of lubricant by forming a layer that allows water to slip smoothly over certain materials - such as the hydrophobic fabric of Olympic swimming suits.

Until recently, however, evidence of nanobubbles has been largely circumstantial because they are so difficult to detect. The bubbles are too small to image with light, and too fragile to probe with most contact techniques that use tiny mechanical probes to measure molecular scale features.

A group at the Ian Wark Research Institute of the University of South Australia (Phil Attard, phil.attard@unisa.edu.au, 618-8302-3564) has now obtained the first direct images of nanobubbles on hydrophobic surfaces. To acquire the images, the researchers gently examined glass surfaces with a tapping-mode atomic force microscope (AFM), which consisted of a conventional AFM probe tip attached to a vibrating cantilever that scanned across samples immersed in water. The groundbreaking images revealed that nanobubbles form closely packed, irregular networks that cover hydrophobic surfaces nearly completely, and that the bubbles rapidly reform after they are disturbed.

The work also seems to have solved a mystery regarding how the miniscule bubbles can exist at all. Pressure inside a bubble is related to the curvature of the bubble's surface - the smaller a spherical bubble is, the higher both the curvature and the pressure must be. High pressures, however, would cause the trapped gasses to rapidly dissolve into the surrounding water, and the bubbles should spontaneously disappear. The tapping-mode AFM resolves this paradox by showing that the nanobubbles are not round, but flattened like pancakes, with curvature and pressure much lower than previously expected. (J. W. G. Tyrrell and P. Attard, Physical Review Letters, 22 October 2001.)

Nobel Prize Addendum

Concerning last week's item on the 2001 physics prize (Update 560), here are a few references to some of the prominent articles concerning Bose-Einstein condensates published in Physical Review Letters in recent years: first lithium BEC, Bradley et al., 28 August 1995; first sodium BEC (Ketterle), Davis et al., 27 November 1995; first hydrogen BEC, Fried et al., 2 November 1998; helium BEC, Pereira Dos Santos et al., 16 April 2001; realization of an atom laser, Mewes et al., 27 January 1997; superfluid properties seen in BEC, Onofrio et al.,11 September 2000; first all-optical BEC, Barrett et al., 2 July 2001.